ARP(9)

NAME
arp, arp_ifinit, arpresolve, arpintr - externally visible ARP functions

SYNOPSIS
#include <netinet/if_inarp.h>
void
arp_ifinit(struct ifnet *ifp, struct ifaddr *ifa);
int
arpresolve(struct ifnet *ifp, struct rtentry *rt, struct mbuf *m, struct sockaddr *dst, u_char *desten);
void
arpintr();

DESCRIPTION
The arp functions provide the interface between the arp module and the network drivers which need arp functionality. Such drivers must request the arp attribute in their "files" declaration.

arp_ifinit()
    Sets up the arp specific fields in ifa. Additionally, it sends out a gratuitous arp request on ifp, so that other machines are warned that we have a (new) address and duplicate addresses can be detected.

    You must call this in your drivers' ioctl function when you get a SIOCSIFADDR request with an AF_INET address family.

arpresolve()
    is called by network output functions to resolve an IPv4 address. If no rt is given, a new one is looked up or created. If the passed or found rt does not contain a valid gateway link level address, a pointer to the packet in m is stored in the route entry, possibly replacing older stored packets, and an arp request is sent instead. When an arp reply is received, the last held packet is send. Otherwise, the looked up address is returned and written into the storage desten points to. arpresolve() returns 1, if a valid address was stored to desten, and the packet can be sent immediately. Else a 0 is returned.

arpintr()
    When an arp packet is received, the network driver (class) input interrupt handler queues the packet on the arpintrq queue, and requests an arpintr() soft interrupt callback. arpintr() dequeues the packets, performs sanity checks and calls (for IPv4 arp packets, which are the only ones supported currently) the in_arpinput() function. in_arpinput() either generates a reply to request packets, and adds the sender address translation to the routing table, if a matching route entry is found. If the route entry contained a pointer to a held packet, that packet is sent.

CODE REFERENCES
The ARP code is implemented in sys/net/if_arp.h, sys/netinet/if_inarp.h and sys/netinet/if_arp.c.

SEE ALSO
ether_ifattach(9)

Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.

STANDARDS
RFC 826

HISTORY
Rewritten to support other than Ethernet link level addresses in NetBSD 1.3.

AUTHORS
UCB CSRG (original implementation) Ignatios Souvatzis (support for non-Ethernet)